The present invention relates to a method of heating a liquid medium by means of a first thermal system and at least one second thermal system following said first thermal system, which thermal systems each have at least one heat exchanger through which the medium flows, and which second thermal system is operated at a higher temperature level than the first thermal system. It also relates to a plant for carrying out the method, including a feed line for feeding the medium to be heated.
Plants in which a liquid medium passes through a plurality of thermal systems in order to be heated, possibly evaporated, are present, for example, in boilers which are heated by flue gas from burners or exhaust gas from gas turbines.
The medium may be water, having additives if need be. Depending on the final load, the water is heated in the boiler to a predetermined temperature in order to be fed, for example, to an industrial plant, a hot-water network, etc., or evaporated in order to be fed, for example, to a steam turbine or an industrial steam load.
The first thermal system in such a boiler, which has a first heat exchanger, a heating-area bank, is normally called the economizer. Due to the temperature conditions, the economizer, which is provided for preheating the feed water in the boiler, preferably works on the flue-gas-side or exhaust-gas-side end of the boiler, i.e. at comparatively low temperatures.
On the other hand, the temperature difference between the flue gas or exhaust gas and the feed water to be heated is relatively small. This in turn results in large heating areas and large heating-area masses associated therewith.
Consequently, an economizer requires a considerable amount of time for adaptation of the temperature, for example during a change in the operational conditions. Furthermore, it is known that there is a risk of dew-point corrosion on account of the temperatures and pressures prevailing in the economizer.
Known methods of raising the feed-water temperature at the boiler inlet or for avoiding dew-point corrosion at the flue-gas-side boiler end, for example as a function of the fuel used, are
recirculation and
bypassing the economizer.
In the case of recirculation, water preheated at the boiler inlet is admixed with the feed water. For the bypassing of the economizer, the feed water bypasses the economizer, and the preheating is carried out in a system working at a higher temperature level, for example a steam-generating system, at the cost of the reduction in the steam generation.
In order not to damage the heating areas, in particular during the start-up or during a change to a sulfurous fuel, measures which go beyond the said measures, i.e. which permit markedly quicker temperature raising in the economizer region, are necessary.
The object of the invention is therefore to provide a method of heating a liquid medium by means of a first thermal system and a second thermal system following said first thermal system and having a higher temperature level, according to which method accelerated raising of the temperature of the first thermal system is made possible under special operating conditions (start-up, fuel change). Furthermore, the risk of dew-point corrosion is to be reduced.
According to the invention, this is achieved in that, for the accelerated raising of the temperature of the medium in the first thermal system, the direct feed of the medium to the same is reduced and in the extreme case prevented, and in that medium flowing through the first thermal system is directed in a circuit.
A plant for carrying out the method according to the invention includes the first thermal system which has a first heat exchanger. The first heat exchanger has an inlet line adjoining the feed line, and an outlet line which runs through a line section to the second thermal system. A first control element is arranged between the feed line and the inlet line. A bypass line, which is equipped with a second control element, runs from the feed line to the outlet line. In addition, a line section runs from the outlet line to the second thermal system. The outlet line is connected to the inlet line through a recirculation line, which has a third control element and a first pump. The recirculation line is arranged paralleel to the first heat exchanger.